1. Field of the Invention
The present invention relates to an installation for sputtering a metallic material on a surface area to be coated. It more particularly applies to the realization of electron emission microtips deposited by vacuum evaporation of a conductive material on the cathode of a flat display screen.
2. Discussion of the Related Art
Microtip screens are mainly constituted by a microtip cathode and a grid that has holes facing the microtips. The cathode faces a catholuminescent anode having a glass substrate that constitutes the display surface.
The operating principle and the detailed structure of such a microtip display are disclosed in U.S. Pat. No. 4,940,916 assigned to Commissariat a l'Energie Atomique.
This device uses the electric field generated between the anode and the cathode so that electrons are extracted from the microtips toward a catholuminescent layer of the anode, subject to the control of the grid.
FIG. 1 is a partial cross-sectional view of the structure of a cathode-grid plate 1 of a microtip screen.
The plate 1 is generally constituted by layers successively deposited on an insulating substrate 2. A conductive layer is deposited over the substrate 2; then, the layer is etched away in a lattice pattern to form cathode conductors 3. A resistive layer 4 is then deposited over the cathode conductors 3. The resistive layer 4 is deposited to protect each microtip 5 against an overcurrent liable to occur at its triggering. An insulating layer 6 is deposited over the resistive layer 4 to insulate the cathode conductors 3 from the grid 7 that is formed by a conductive layer. Holes 8 are provided in layers 6 and 7 to accommodate the microtips.
The deposition of microtips 5 is conventionally carried out by sputtering a conductive material, for example molybdenum. A lift-off layer, for example nickel, is previously formed over the surface of grid 7, and also on the edges of holes 8. The material evaporated at these places, which constitutes the microtips, can thus be eliminated.
A problem encountered in the formation of microtips is that the conventional devices for sputtering a metallic material are not adapted to achieve depositions with a practically normal incidence over large-size plates.
As represented in FIG. 1, the microtips 5 should be deposited at the bottom of holes 8, but the evaporated material should neither fill holes 8, nor be deposited on the walls of the holes in the insulating layer 6. Otherwise, the excessive conductive material could not be eliminated selectively with respect to microtips 5. The lift-off layer that is generally used coats only the surfaces of the grid layer 7. The incidence of the sputtering beam should be practically normal with respect to the plate 1 to prevent deposition outside the desired areas.
Moreover, a screen plate 1 is constituted by a single piece. Hence, the practically normal incidence of the sputtering beam should be obtained over the whole surface of the plate.
FIG. 2 schematically represents a conventional device for evaporating a metallic material such as the one used to form microtips 5.
A metallic material 10 is vacuum evaporated in a crucible 11 containing the material to be deposited. The content of the crucible 11 is heated and sputtered with an electron bombardment device (not shown). Conventionally, a sputtering device comprises a turntable 12 moving in an epicycloidal way. The plates 1 to be coated are suspended on the inner surface of the turntable 12 which is concave towards the crucible 11. A sputtering source, constituted by crucible 11 and the electron bombardment device, is disposed in the middle of turntable 12. The rotation of turntable 12 and plates 1 provides homogenous deposition.
The need for a sputtering beam having a practically normal incidence angle with respect to the plane of plates 1 requires, for large-size plates, the sputtering source to be disposed far away from the plates. This is necessary for the maximum incidence angle .alpha. of the sputtering beam which reaches a determined plate to be very small. In other words, the turntable 12 should have an important radius so that the cone of the sputtered material, whose base reaches a determined plate 1, has a very small angle. Typically, the maximum incidence angle of a beam on a determined plate 1 should range from 5.degree. to 15.degree.. For example, for a 20.times.30-centimeter rectangular plate 1, with a maximum incidence angle of approximately 9.degree., the distance between the plates 1 and crucible 11 is approximately 1 meter.
Such a distance requires very large-size devices.
Another drawback of the prior art is that the loading and unloading steps of a new series of plates 1 in turntable 12 pollutes the vacuum in which sputtering is achieved, which decreases the production rate of the device.